In a wireless cellular communication system, a wireless access network includes two main parts which are a base station system and a wireless network controller respectively, wherein the design of the base station system directly influences the networking mode of the wireless network, the covering quality of the network and the operation cost of constructing the network.
The importance of the design of the base station system will be analyzed with an example of the TD-SCDMA system.
FIG. 1 is a schematic diagram showing a configuration of a conventional base station. As shown in FIG. 1, in the TD-SCDMA system, Node B is connected with a radio network controller (RNC) through a standard Iub interface and Node B includes two parts which are located indoors and outdoors respectively.
The outdoor part of Node B includes an amplifier unit (PA (power amplifier) & LNA (low noise amplifier)) and an antenna (ANT), wherein the PA & LNA receives an analog RF signal outputted from the indoor part of Node B through a RF cable, amplifies the signal and transmits the signal through the ANT; or the PA & LNA receives and amplifies an analog RF signal from the ANT and transmits the analog RF signal to a RF processing unit (RFU) of the indoor part of Node B through the RF cable.
The indoor part of Node B includes five parts:
a RNC interface unit (RIU), for performing an interface function between Node B and the RNC; a central control unit (CCU), for performing a main control function of Node B and a function of switching signaling and service data between units of Node B, wherein data from the RNC is transmitted to the CCU through the RIU and data to be transmitted upwardly in Node B is transferred to the RNC through the RIU; a digital baseband unit (BBU), for performing a processing function of the digital signal at the symbol level and the chip level at the physical layer and for intercommunicating a digital baseband signal with the RFU; the RFU is for performing conversion between a digital baseband signal and an analog RF signal; and a GPS unit (GPSU) and a GPS antenna (GPS ANT), for obtaining a global position system (GPS) clock signal through the GPS ANT, generating a clock signal and providing the generated clock signal to other units in the Node B.
The RFU includes a digital intermediate frequency (IF) module and a RF transceiver module. FIG. 2 shows the modules of the RFU shown in FIG. 1 and their functions. As shown in FIG. 2, the BBU is connected with the digital IF module of the RFU and the digital baseband signal is transmitted between the BBU and the digital IF module of the RFU; the digital IF module of the RFU is connected with the RF processing module of the RFU and the analog IF signal is between the digital IF module of the RFU and the RF processing module of the RFU; the RF processing module of the RFU is connected with the PA&LNA and the analog RF signal is between the RF processing module of the RFU and the PA&LNA through the RF cable. The detailed functions are as follows.
Through the uplink path of the RFU, the RFU receives the analog RF signal transmitted from the PA&LNA, processes the received analog RF signal and the digital baseband signal to the BBU. The processing of the RFU on the received analog RF signal includes: (1) the RF transceiver module of the RFU performs the amplifying, filtering, gain control, conversion from the analog RF signal to the analog IF signal on the analog RF signal received through the antenna, performs the amplifying, filtering and gain control on the analog IF signal obtained after the conversion and finally transmits the processed analog IF signal to the digital IF module of the RFU; (2) the digital IF module of the RFU performs the A/D conversion, digital down-conversion and baseband signal shaping and filtering processing on the received analog IF signal to obtain the digital baseband signal and transmits the digital baseband signal to the BBU.
Through the downlink path of the RFU, the RFU receives the digital baseband signal from the BBU, processes the received digital baseband signal, and transmits the analog RF signal to the PA&LNA. The processing of the RFU on the received digital baseband signal includes: (1) the digital IF module of the RFU performs the baseband signal shaping and filtering processing, digital up-conversion and D/A conversion on the received digital base station signal to generate the analog IF signal and transmits the generated analog IF signal to the RF transceiver module of the RFU; (2) the RF transceiver module of the RFU performs the amplifying, filtering, gain control, conversion from the analog IF signal to the analog RF signal on the received analog IF signal, performs the amplifying, filtering and gain control on the analog RF signal obtained after the conversion and finally transmits the processed analog RF signal to the PA&LNA.
In the conventional base station system, since the RFU is located at the indoor part of the base station and there is requirement on the distance between the RF transceiver module and the antenna, which makes the distance between the indoor part of the base station and the antenna limited to scores of meters. In addition, since the area covered by the base station is decided by the RF transceiver module while the RF transceiver module of the conventional base station belongs to the indoor part, when the wireless communication needs to cover larger and larger area, there should be many base station sites so as to cover different areas with the conventional base stations, therefore, the network operation cost increases. In addition, since the design of the conventional base station makes the indoor part and the outdoor part of the base station transmitting signals through the RF cable and the RF cable is thick and heavy, which brings difficulty in mounting the RF cable. Especially, if the base station adopts the smart antenna, multiple RF cables should be mounted, which adds difficulty in mounting and cost.